This invention relates to a system for demodulation of phase shift keying signals (hereafter referred to as PSK signals) used in data transmission systems. More particularly, this invention contemplates, in a system which extracts reference carrier wave and improves the signal-to-noise ratio of the reference carrier wave by use of a bandpass filter, a circuit for automatically compensating for the degradation of demodulation characteristics due to variations in the input frequency to the system by eliminating modulated components of PSK signals by means of a remodulator or by eliminating the modulated components by means of an N-frequency multiplier (in the case of N-phase PSK).
In the PSK communication, the data is transmitted in the form of relative phase changes of the carrier wave. Demodulation of such PSK signals is effected by comparing the phase of the PSK signal and that of the reference carrier wave. In the demodulation of PSK signals, therefore, the receiver is required to regenerate the reference carrier wave from the received signal. In one known method available for the extraction of the reference carrier wave from the PSK signal, the PSK signal is converted into a continuous signal by use of a remodulator or an N-frequency multiplier (in the case of N-phase PSK modulated signal) to thereby eliminate modulated components and further phase jitters are reduced by means of a bandpass filter. This method operates with a relatively simple configuration and proves excellent in the sense that the reference carrier wave suffers little from jitters. Nevertheless, it is deficient in that it is vulnerable to changes in the input frequency. To be specific, since the bandpass filter has its central frequency fixed, a possible change in the input frequency will cause a phase modulation in the regenerated carrier wave, with the result that the demodulated output of the PSK signal is attenuated.
One conventional system (described in "The Design of a PSK MODEM for Telesat. TDMA System", S. Yokoyama et al, ICC 75, June 16-18, San Francisco) known to improve the situation detects variations in the input frequency during the regeneration of the reference carrier wave and compensates for the phase of the regenerated carrier wave by use of the detected signal. Generally, however, it is not easy to effect perfect detection of variations in the input frequency. Particularly where the input frequency is high, a possible delay in the pass time of the remodulator or the N-frequency multiplier and a possible delay in the pass time of the bandpass filter may cause an erroneous detection.